Most of the arguments pro and con for an intervention in Syria have already been made.
I think the consensus is that while stopping Assad in 2011 might have been wise (before the use of the WMD and 100,000 dead), doing so now is, well, problematic.
He has shown far more resilience than the administration thought when it ordered him to leave (dictators rarely leave when ordered to by an American president). The opposition seems far more dominated by al-Qaeda affiliates than originally thought (not all that many Westernized intellectuals, persecuted minorities, and Arab Spring bloggers are still left on the barricades).
In addition, both critics and supporters of the president point out that had Obama just kept quiet, he could have kept the option of intervening on his own timetable, rather than being forced to when his rhetorical red lines were not merely crossed but erased in humiliating fashion. Since his bluff has been called, he now has to act to save face rather than to save lives — 100,000 of them too late.
Yet the rub is not just that it is unlikely that we can find all the WMD depots and destroy them safely from the air (keeping them out of both Assad’s and our allies’ hands).
Nor is the problem just that it is unlikely that a limited punitive blow against Assad will topple him (and then what?) and restore American rhetorical credibility.
Instead, we are not sure that the opposition is likely to be any better than the monster Assad. Did we learn nothing from Libya and Egypt? The paradox in the Middle East is that Americans can control the postwar landscape and promote consensual government only by inserting large numbers of ground troops — an unacceptable political reality. A Putinesque shelling and bombing solution (more rubble, less trouble) is ethically unacceptable to most Americans.
Then there are the domestic politics. During the Iraq War, authorization from Congress was essential; now it is not? The excruciating and ultimately failed effort in 2002 at the UN took weeks; now it is not even attempted by a Peace Prize laureate? Bombing a monstrous regime guilty of past WMD use was amoral; now it is ethical?